that one big determining factor in which class you end up is the choices you make.

I think the biggest factor is birth. If there's choice involved, it may make a difference between being poor working class and "middle class" working class, but outside of a few superachievers, it's unlikely to vault you into another class. The reverse is also true - barring some really disastrous choices, a person born into wealth is unlikely to fall down to the middle or working classes.